Microlithographic projection exposure methods and systems (machines) are currently used to fabricate semiconductor components and other finely patterned components. A microlithographic exposure process involves using a mask (reticle) that carries or forms a pattern of a structure to be imaged. The pattern is positioned in a projection exposure system between an illumination system and a projection objective in a region of the object surface of the projection objective. Primary radiation is provided by a primary radiation source and transformed by optical components of the illumination system to produce illumination radiation directed at the pattern of the mask in an illuminated field. The radiation modified by the mask and the pattern passes through the projection objective, which forms an image of the pattern in the image surface of the projection objective, where a substrate to be exposed is arranged. The substrate normally carries a radiation-sensitive layer (photoresist).
When a microlithographic projection exposure system is used in the manufacture of integrated circuits, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the integrated circuit. This pattern can be imaged onto an exposure area on a semiconductor wafer which serves as a substrate.
In one class of microlithographic projection exposure systems each exposure area is irradiated by exposing the entire pattern of the reticle onto the exposure area at once. Such apparatuses are commonly referred to as wafer steppers.
In alternative exposure systems commonly referred to as step-and-scan apparatus or wafer scanner, each exposure area is irradiated progressively in a scanning operation by moving the mask relative to an illumination beam in an effective object field of the projection objective, and simultaneously moving the substrate relative to the projection beam in the conjugate effective image field of the projection objective in respective scanning directions. The mask is typically held in place by a mask holder, which is movable parallel to the object surface of the projection objective in a scanning apparatus. The substrate is typically held by a substrate holder, which is movable parallel to the image surface in a scanning apparatus. The scanning directions may be parallel to each other or anti-parallel to each other, for example.
The projection objective defines, in general, an optical axis with reference to which the optical elements that belong to the projection objective are arranged. Generally, these optical elements are rotationally symmetrical relative to this optical axis, and the optical axis is a normal to the object field and image field. In this case, the design of the projection objective is denoted as being rotationally symmetrical. Such a rotationally symmetrical design does not in general imply that all optical elements in the physical implementation of the projection objective desirably is designed to be rotationally symmetrical. In the case of folded projection objectives, and/or in the case of catadioptric projection objectives, with on-axis object field or off-axis object field, the foldings and penetrations of the beam path may involve cutouts in optical elements, and/or optical elements that serve solely for beam deflection such as, for example, folding mirrors, and which have no refractive power.
During the exposure of the wafer, the projection exposure machine is operated with a prescribed image-side numerical aperture NA, also termed “numerical aperture” below for short, and with a setting (illumination setting) prescribed by the illumination system, for example an incoherent, annular, dipole or quadrupole illumination setting. The numerical aperture NA is defined by the position and the diameter of a diaphragm (aperture stop) in the projection objective. Common numerical apertures for projection objectives for microlithography are 0.5, 0.6, 0.7, 0.8, or 0.9 and values in between, for example. In the case of projection exposure machines that are designed for immersion operation, the numerical aperture may increase by approximately 50%, for example. The illumination setting is generally prescribed by optical elements of the illumination system such as the diffractive optical elements.
At each instant of the exposure, a maximum radiation bundle cut by the diaphragm passes from the effective object field to the effective image field from each field point belonging to the effective object field. In an ideal projection objective whose aberrations (also referred to as imaging errors) are determined only by the design, the wavefront, defined by this maximum radiation bundle, in the vicinity of the image point belonging to the object field point corresponds approximately to a spherical wave with the image point as midpoint. Such projection objectives are therefore also termed diffraction limited.
In general, during the exposure the projection objective is aligned with its optical axis in the direction of gravity. The generally planar reticle is in this case aligned horizontally, i.e. parallel to the object field of the projection objective. A consequence of this is that the reticle sags owing to the force of gravity, the sagging being a function of the type of the reticle and of the mounting technique securing the reticle, and is not known a priori or is difficult to determine. This deformation of the reticle caused by sagging of the reticle has the consequence that the position of the individual locations on the reticle that are to be imaged are displaced in a way that cannot be completely predicted a priori, the direction and length of this displacement themselves in turn being a function of the location on the reticle. What is involved here are pure spatial displacements that are not accompanied by a pupil error, or accompanied only by a vanishingly small pupil error.
A similar situation may be caused on the image side, e.g. on the side of the wafer. The problem may be of lesser importance there, since a wafer can typically be supported from below. Such a support is difficult to obtain for the reticle, as a matter of course.
A further cause for possible sagging of the reticle is the direct influence on the reticle of its mounting technique. Here, there generally occur forces and moments caused by bearings and/or clamps acting on the reticle. These are, a priori, likewise not fully known, and can differ from reticle to reticle, but they can also be identical for classes of reticle.